A Si atom is removed from a SiC surface of a SiC power device when an activation heat treatment is performed in a step of forming an impurity layer, so that the SiC surface is roughened. When the Si atom is removed, a carbon rich layer is formed on the SiC surface. It is necessary to remove the carbon rich layer for reducing a leak current and for protecting the device from generating failure. Accordingly, to remove the carbon rich layer, a step for forming a sacrifice oxidation film and a step for removing the sacrifice oxidation film are added.
The step for forming the sacrifice oxidation film is a thermal oxidation process. In the thermal oxidation process, a region in which an impurity is implanted is oxidized with a thermal oxidation speed, which is much different from that of a region in which no impurity is implanted. Specifically, the thermal oxidation speed in the implanted region is larger than that in the non-implanted region so that the implanted region is oxidized rapidly. Accordingly, the thickness of the thermal oxidation film in the implanted region is larger than that in the non-implanted region.
Thus, when the sacrifice oxidation film is removed in a HF acid process, the implanted region provides a concavity (a constricted part), compared with the non-implanted region. This is because the thickness of the thermal oxidation film in the implanted region is large. This concavity may cause deviation of thickness in a step for forming a gate oxide film. Thus, reliability of the gate oxide film is reduced.
To improve the above difficulty, a method for reducing surface roughness is disclosed in, for example, JP-A-2005-260267. Specifically, an organic film such as a photo resist is patterned, and then, an impurity ion is implanted. After that, the organic film is carbonized so that a graphite film is formed. The graphite film is used for a mask in an annealing process with high temperature.
In the above method, the graphite film functions as a mask, and thereby, the surface roughness under the mask is improved.
Another method for improving the surface roughness is disclosed in JP-2005-303010. Specifically, after a drift layer is epitaxially grown, a Si atom is sublimated in a vacuum high temperature anneal process, so that a homogeneous carbon layer is formed. The carbon layer is used for a cap layer in an activation process, in which an impurity layer is annealed so that the impurity layer is activated. Thus, when the carbon layer is used, an impurity included in organic solvent is not diffused into the SiC substrate. This feature is similar to the graphite film. Thus, device characteristics are not deteriorated.
However, in the method disclosed in JP-A-2005-260267, the graphite film is made of the organic film having a predetermined pattern, which is used for ion implantation. Therefore, a region corresponding to the opening of the organic film for the ion implantation is not covered with the graphite film.
Thus, the Si atom is sublimated in the anneal step with high temperature from the opening that is not covered with the graphite film, and thereby, the carbon rich layer is formed in the opening. Accordingly, when the device is formed, the carbon rich layer is removed in the sacrifice oxidation process. The step for forming the sacrifice oxidation film and the step for removing the sacrifice oxidation film are added. Thus, the concavity is produced from rapid oxidation.
In the method disclosed in JP-A-2005-303010, after the carbon layer is formed, a SiO2 film is formed on the carbon layer in order to perform selective ion implantation. After the film is processed by a photo etching step, the ion implantation step is performed.
When the SiO2 film is formed on the carbon layer made of amorphous film, adhesiveness between the SiO2 film and the carbon layer may be insufficient. Thus, when the SiO2 film is processed to have a fine pattern, the SiO2 film as the mask may be removed (i.e., may peel off) from the carbon layer. Thus, the SiO2 film does not function as the mask sufficiently. Thus, the device performance may be reduced.
JP-A-2005-303010 teaches that ion implantation step is performed after the carbon layer is formed. Thus, the SiO2 film is formed on the carbon layer, and the SiO2 film is patterned in a photo etching process. Then, the ion implantation step is performed.
However, when the SiO2 film is formed on the carbon layer as an amorphous layer, the adhesiveness between the SiO2 film and the carbon layer is not sufficient. Thus, when the SiO2 film is patterned with a fine pattern, the SiO2 film as a mask is removed from the carbon layer. Thus, the SiO2 film does not function as a mask for the ion implantation step, so that the performance of the device is reduced.
Further, in JP-A-2005-303010, after the ion implantation step is performed in order to form an impurity layer, the carbon layer is formed. After the drift layer is epitaxially grown, a Si component on the surface of the drift layer is sublimated so that the carbon layer is formed on the surface of the drift layer. In this case, by performing a series of steps, the carbon layer is formed. However, it is necessary to add the ion implantation step before the carbon layer is formed.
Further, when the carbon layer is formed after the ion implantation step, an impurity region is formed together with the carbon layer. Thus, in the impurity region, crystal structure is distorted. Accordingly, when the carbon layer is formed at a high temperature in a range between 1100° C. and 1400° C., carbonizing speed (or sublimation speed of the Si atom) in the impurity region is different from the SiC surface portion having proper crystal structure. Accordingly, when the carbon layer is removed, a concavity is formed, which is similar to the sacrifice oxidation step and the step for removing the sacrifice oxidation film. Thus, the thickness of the gate oxide film is deviated, and the reliability of the gate film is reduced.